A salted battery: NMR by-products

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  • Published: Nov 15, 2016
  • Author: David Bradley
  • Channels: NMR Knowledge Base
thumbnail image: A salted battery: NMR by-products

Lifecycle improvements

Sodium-oxygen batteries have improved cycle life due to highly concentrated electrolytes. NMR used to test byproducts in usage Credit: Wiley

Sodium-oxygen batteries have much improved cycle life due to highly concentrated electrolytes, according to US researchers who have used nuclear magnetic resonance (NMR) spectroscopy to identify the discharge by-products.

The invention of lightweight rechargeable batteries is the great enabler of the modern mobile world whether you carry a smart phone, a laptop or notebook computer or drive an electric car. They will also play a significant role in the storage of electricity generated by intermittent or periodic but renewable power sources such as wind and solar power. Whatever the device in question, we need efficient and long-lived batteries to make the most of the technology and to pave the way to future yet to be invented technologies. Unfortunately, as anyone who uses current rechargeable batteries perhaps knows, they can be bulky, heavy, short-lived when demand is high and come to the end of their useful life in far too short a time.

Until we find an entirely novel approach to storing power and producing electricity when we are on the go, materials scientists are focusing on bringing current understanding and materials to what is essentially a nineteenth century invention. Indeed, the search for a next-generation battery has been focused in recent years on sodium– oxygen batteries rather than the all too familiar lithium-ion batteries that power so many of our gadgets. Theoretically at least, a sodium-oxygen battery would be able to achieve an efficiency level with a high energy density that is simply unattainable with the current battery architectures and materials. However a practical implementation of this battery chemistry has not yet been found.

Reverse engineering

Now, chemist Yiying Wu of the Ohio State University, in Columbus, Ohio, USA, who invented the superoxide-based potassium-oxygen battery has turned his attention to its sodium-based chemical cousin and demonstrated that by using a highly concentrated electrolyte solution it might be possible to make the sodium-oxygen battery even more stable, and therefore more practicable. The team, which includes Mingfu He, Xiaodi Ren, Neng Xiao, William D. McCulloch, and Larry A. Curtiss of the Materials Science Division, at Argonne National Laboratory in Lemont Illinois and Kah Chun Lau of California State University at Northridge, report details of their experiments and analyses of a sodium-oxygen battery in the journal Angewandte Chemie.

The concept of alkali metal-oxygen batteries holds great promise, simply because their theoretical energy density is very high. In this battery setup, one electrode is made from the pure alkali metal. When the charged battery is discharged, the alkali metal electrode releases electrons to drive the current in the circuit and positive ions are lost to the electrolyte. With a counter electrode made of porous carbon and in contact with the air, oxygen (from the air) is reduced by the electron flux in the presence of the positive metal ions. Hypothetically this would generate a number of different metal oxide compounds. However, when the battery is charged the process would be reversed by an applied electric current so that oxygen is once more released to the air at the positive electrode and the alkali metal ions are deposited at the negative electrode. Optimising this reverse process would depend strongly on the products formed in the discharge step and whether or not they were then amenable to being broken down to cleanly release the oxygen and their metal ions.

Removing obstacles

Indeed, this is one of the fundamental obstacles blocking the practical implementation of such a battery, because numerous side reactions can take place stymieing stability and retarding reversibility. In trials with lithium as the alkali metal, the pores can become clogged by lithium peroxide. Sodium is more readily obtainable anyway, does not generate sodium peroxide, instead sodium superoxide forms, which can almost completely revert to the elements during charging.

To work effectively though, the system also needs an anhydrous, aprotic solvent. Dimethylsulfoxide (DMSO) would usually the solvent to which researchers would turn first for electrochemical applications but it reacts with sodium to form products interfere with the battery electrochemistry. As such, the team has used in addition to DMSO a very high concentration of the organic salt sodium trifluoromethanesulfonimide (NaTFSI) which stabilizes the solvent in the presence of sodium.

The researchers built a small prototype battery with this chemical system. They demonstrated good electrochemical properties and showed that it could undergo at least 150 charge-discharge cycles without any notable loss of efficiency. One with a dilute electrolyte solution lasted a mere six cycles. The team then used Raman spectroscopy of the NaTFSI/DMSO electrolyte solutions in conjunction with computational simulations to help them explain how the salt stabilises the solvent. They say that the highly concentrated solution results in a structure of loosely cross-linked Na(DMSO)3TFSI units that binds to a large proportion of the DMSO molecules, leaving only a few available for reaction. The sodium then preferentially attacks the TFSI anions, forming a passivating protective layer on the sodium electrode. The NMR studies of a heavy water extract of the discharge cathode revealed the presence of sodium acetate, sodium formate, and sodium fluoride, which goes some way to explain why approximately 18% of the discharge capacity does not lead to sodium superoxide.

Related Links

Angew Chem Int Edn Engl 2016, 55,online: "Concentrated Electrolyte for the Sodium–Oxygen Battery: Solvation

Structure and Improved Cycle Life"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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